Conductivity and Total Dissolved Solids (TDS) are related parameters commonly used to characterize the quality of water. While they measure different aspects of water composition, there is a correlation between them, especially in the context of dissolved ions and minerals. Here's how conductivity and TDS are related:

1. Conductivity:

   • Conductivity measures the ability of a solution to conduct an electric current. It is influenced by the concentration of ions (charged particles) dissolved in the water.
   • In pure water, conductivity is very low because there are few ions present to carry the electric current.
   • The presence of dissolved ions increases the conductivity of water, as these ions facilitate the flow of electric charge.

2. Total Dissolved Solids (TDS):

   • TDS measures the total concentration of dissolved solids in water, including organic and inorganic substances such as salts, minerals, metals, and other compounds.
   • TDS is typically measured in milligrams per liter (mg/L) or parts per million (ppm) and is often determined by evaporating a known volume of water and weighing the residue left behind.

Relation between Conductivity and TDS:

• There is a strong correlation between the conductivity of water and its TDS level, particularly in waters containing primarily inorganic dissolved solids.
• In general, as the concentration of dissolved ions and solids increases, so does the conductivity of the water.
• While conductivity measures the ability of water to conduct electricity, TDS quantifies the concentration of dissolved solids that contribute to conductivity.
• The relationship between conductivity and TDS can be expressed using empirical conversion factors, which vary depending on the composition of the dissolved solids and the temperature of the water.
• Common conversion factors used to estimate TDS from conductivity readings include:
  • For freshwater: TDS (ppm) ≈ 0.5 × Conductivity (μS/cm)
  • For saline water: TDS (ppm) ≈ 0.7 × Conductivity (μS/cm)

Factors Affecting the Relationship:

• The relationship between conductivity and TDS may vary depending on factors such as:
  • Temperature: Conductivity increases with temperature, so the relationship between conductivity and TDS may change at different temperatures.
  • Composition of Dissolved Solids: Different types of dissolved solids (e.g., salts, organic matter) may have different effects on conductivity, leading to variations in the conductivity-TDS relationship.

Limitations:

• While conductivity is a useful indicator of water quality, it does not provide information about the specific types of dissolved solids present in the water. TDS measurements offer more detailed information about the composition of dissolved solids.
• The conductivity-TDS relationship may not be precise in all cases, especially in waters containing significant amounts of organic matter or non-conductive substances.

In summary, conductivity and TDS are related parameters that provide valuable information about the quality and composition of water. While conductivity reflects the water's ability to conduct electricity due to the presence of dissolved ions, TDS quantifies the total concentration of dissolved solids. Understanding the relationship between conductivity and TDS can help in interpreting water quality data and assessing the suitability of water for various purposes.